The present invention relates to a thick-film thermal printhead. Specifically, the present invention relates to a thick-film thermal printhead including a very hard glass layer for protection of the heating resister. Further, the present invention also relates to a method for manufacturing such a thick-film thermal printhead as the above.
An example of a prior art thick-film thermal printhead is shown in FIG. 5. The illustrated thermal printhead includes an insulating substrate 51, a glaze layer 52 formed on the substrate 51 for heat reservation, and a conductor pattern 53 formed on the glaze layer 52. The Conductor pattern 53 includes a common electrode, individual electrodes and so on. The thermal printhead further includes a heating resister 54 electrically connected with the conductor pattern 53, and a first glass coat layer 55 for protection of the heating resister 54, the conductor pattern 53 and the glaze layer.
In addition to these constituent elements described above, the above prior art thermal printhead further includes a second glass coat layer 56 formed on the first glass coat layer 55.
The second glass coat layer 56 is made of a highly strong glass material. Such an arrangement as described above is adopted in order to provide reliable protection to the heating resister 54 and others.
According to the prior art thermal printhead, the heating resister 54 is formed by first printing and then baking a predetermined resister paste on the glaze layer 52. Specifically, the paste material is a mixture of ruthenium oxide, a glass frit and a solvent. The glass frit has an average grain size of about 5 xcexcm.
The first glass coat layer 55 is formed for example of an amorphous lead glass containing about 26.5% resin material and about 73.5% glass material. A glass paste for forming the glass layer 55 is a mixture of a glass frit and a solvent. The glass frit has a maximum grain size of about 10 xcexcm.
The prior art thermal printhead is known to have a problem in the following point. Specifically, as described as above, the average grain size of the glass frit contained in the resister paste is about 5 xcexcm. The heating resister 54 made from such a resister paste has a surface roughness expressed as centerline average roughness Ra of about 0.6 xcexcm, which is a relatively large value. Next, the maximum grain size of the glass frit contained in the glass paste is about 10 xcexcm as has been described.
The glass coat layer 55 made from such a glass paste has a surface roughness expressed as the centerline average roughness Ra of about 0.2 xcexcm, which is a relatively large value.
As will be understood easily, if the centerline average roughness Ra on the surface of the heating resister 54 has a large value, the centerline average roughness Ra on the surface of the first glass layer 55 also has a large value (i.e. the first glass coat layer 55 has a poor state of surface). Under such a circumstance, if the second glass coat layer 56 is subjected to an impact force and so on, there is a possibility that stress concentration occurs in a specific location of the second glass coat layer 56. As a result, the second glass coat layer 56 may develop a crack for example, or the second glass coat layer 56 may flake off the first glass coat layer 55.
An object of the present invention is to provide a thick-film thermal printhead capable of eliminating or reducing the problem described above. In order to achieve the object, the present invention makes use of the following technical means.
A thermal printhead provided by a first aspect of the present invention comprises an insulating substrate, a heating resister formed on the substrate, a first glass coat layer covering the heating resister and formed on the substrate; and
a second glass coat layer formed on the first glass coat layer, wherein the heating resister has a centerline average roughness not greater than 0.3 xcexcm.
According to a preferred embodiment of the present invention, the first glass coat layer has the centerline average roughness not greater than 0.1 xcexcm.
Preferably, the heating resister is formed from a paste material containing a glass frit having an average grain size not greater than 2 xcexcm.
Further, the first glass coat layer may be formed from a paste material containing a glass frit having an average grain size not greater than 1.5 xcexcm.
Preferably, the glass frit has a maximum grain size not greater than 6 xcexcm.
According to a second aspect of the present invention, there is provided a method for making a thermal printhead including an insulating substrate, a heating resister formed on the substrate, a first glass coat layer covering the heating resister and formed on the substrate, and a second glass coat layer formed on the first glass coat layer. The method comprises the steps of forming the heating resister on the substrate, forming the first glass coat layer, covering the heating resister, and on the substrate, and forming the second glass coat layer on the first glass coat layer, wherein the heating resister is formed from a paste material containing a glass frit having an average grain size not greater than 2 xcexcm.
According to a preferred embodiment of the present invention, the above method further includes a step of printing and baking the paste material.
Preferably, the first glass coat layer is formed from a paste material including a glass frit having an average size not greater than 1.5 xcexcm. Preferably, the glass frit has a maximum grain size not greater than 6 xcexcm.
The second glass coat layer can be formed by spattering.
Other features and advantages of the present invention will become clearer from an embodiment to be described with reference to the attached drawings.